Differential pressure sensors convert pressure fluctuations into electrical signals using piezo-electric or semi-conductor sensors. Early sensors were mechanical and could handle higher pressure fluctuations but were not as accurate and did not output digital data. The modern sensors are more fragile and require overpressure protection.
Over-pressure protection and under-pressure occur frequently in an input/output system or hydraulic system under pressure. Pressure fluctuations, especially in incompressible fluids, can change very quickly. These fast and repeated pressure fluctuations require durable and responsive protection systems.
The disclosure of U.S. Pat. No. 4,072,058 to Whitehead provides a pressure sensor protector that includes two pairs of chambers and two pairs of diaphragms. The first two diaphragms on the inlet and outlet ends of the protection device deform in over or under-pressure conditions to cut off flow to the internal chambers. Since these deformations can be slow, the protection device contains a second internal pair of diaphragms. The first diaphragms define a first pair of chambers and the second diaphragms define a second central chamber, each chamber provided with fluid channels which transfer pressure from one side to the next through a fill fluid.
In an over-pressure situation, the inlet side diaphragm would compress against the wall of the first chamber and cut off flow. However, the deformation increases the pressure in the chamber and forces the internal diaphragms to deform. This equalizes pressure and, ideally, protects against the pressure surge. The device also provides pressure bypasses between chambers to speed the equalization. A similar fluid-filled overpressure protector with fluid bypass channels is disclosed in U.S. Pat. No. 4,329,877 to Hershey.
However, the fill fluids (usually incompressible fluids) present several disadvantages and challenges. First, fill fluids that escape may contaminate the larger system or create explosion hazards. Second, the fill fluid is subject to expansion and contraction with temperature, such that the temperature range over which the unit can remain calibrated is limited. Third, compressibility of the fill fluid further limits the maximum line pressure that the unit can operate at.
In fluid filled designs, even after the overload protection engages, a fraction of the overload pressure will continue to be seen by the sensor. This is due to the unintended deformation of the internal components from the increasing overload condition. This is most severe when the sensor is a miniature silicon piezo-resistive or silicon capacitive type of low capacity since they are sensitive to minute volume changes.
Additionally, fluid-filled designs can inadvertently allow permeation by small molecules such as hydrogen which affect the calibration and accuracy of the sensor. Also, fluid-filled designs are inherently more expensive to build and require extensive measures to degas and properly fill. Thus, they also require higher skill levels to assemble and adjust for performance.
The disclosure of U.S. Pat. No. 4,668,889 to Adams discloses a non-fluid-filled design which uses biasing springs in the internal chamber to replace the fill fluid. This arrangement allows the device to be scaled down further than other designs while maintaining durability. However, this disclosed design would require fluid flow restrictors in order to allow the system to react quickly enough. These flow restrictors would also limit the frequency response of the sensing element and would not protect against larger flow rates. Accordingly, the device disclosed herein addresses the issues to provide a scalable, fast acting redundant over and under pressure protection system.